AcronymsAKAAuthentication and Key AgreementASApplication ServerAVPAttribute Value PairCCBSCall Completion on Busy SubscriberCCNRCall Completion on No ReplyCCNRegCall Completion Service to a user who is notregistered or not availableHSSHome Subscriber ServerI-CSCFInterrogating-Call Session Control FunctionIMSIP Multimedia SubsystemIPInternet ProtocolISUPISDN (Integrated Service Digital Network) UserPartLIALocation Info AnswerLIRLocation Info RequestMAAMultimedia- Authentication -AnswerMARMultimedia- Authentication -RequestMGCFMedia Gateway Controller FunctionMRFMedia Resource FunctionNGNNext Generation NetworkP-CSCFProxy-Call Session Control FunctionPLMNPublic Land Mobile NetworkPPAPush-Profile-AnswerPPRPush-Profile-RequestPS(SIP) Proxy ServerPSTNPublic Switched Telephone NetworkREL(ISUP) Release MessageRLC(ISUP) Release Complete MessageRTARegistration Termination AnswerRTPReal-Time Transport ProtocolRTRRegistration-Termination-RequestSAAServer-Assignment-AnswerSARServer-Assignment-RequestS-CSCFServing-Call Session Control FunctionSDPSession Description ProtocolSIPSession Initiation ProtocolSLSubscriber LocatorTCAPTransaction Capabilities Application PartUAAUser-Authorization-AnswerUARUser-Authorization-Request
1. Field of the Invention
The present invention generally relates to call completion services which enable call attempts to be completed by a telecom network without the calling users having to initiate (repeated) new attempts themselves. It peculiarly concerns calls which are to set up through a SIP based telecom network, in particular the IP Multimedia Subsystem.
2. Description of the Prior Art
Examples of call completion services today include, Call Completion to a Busy Subscriber (CCBS), and Call Completion on No Reply (CCNR).
The network elements monitor the state of the called user, and ‘recalls’ the calling user, when the called user becomes ‘free for recall’. In case of multiple calling users making call attempts to the same called user, a called user queue is used to store the relevant info, and then complete the call attempts in sequence. (Responsibility: terminating switch). A calling user can invoke a ‘call completion’ service multiple times to different called users—this is also managed by using a queue for the calling user. (Responsibility: originating switch)
CCBS and CCNR are today available in the Public Switched Telephone Network (PSTN) and in the Public Land Mobile Network (PLMN), for users that are busy or that are absent when their phone terminals are called. Call completion within a PLMN, is available for the not-reachable case. With the IP based telecom networks appeared new concepts: registration and presence. In an IP based network, a user cannot be called if the user is not “registered” in a registrar server. Similarly, a user cannot benefit from presence based services, if this user has not a status “available” in a presence server.
As concerns users that are busy or absent when their phone terminals are called, CCBS and CCNR for the Next Generation Networks (NGN), and for the Internet Protocol Multimedia Subsystem (IMS) based on the Session Initiation Protocol (SIP), are being discussed in standardization bodies (ETSI TISPAN). CCBS and CCNR services for SIP-based (not necessarily IMS) networks are being discussed in IETF, and drafts have already been proposed.
As concerns users that are not registered/not available, today there is no Call Completion to a Not Registered/Not available user (CCNReg) service, enabling communication attempts to a called IMS/SIP user who is not registered/not available, to be completed without the calling user having to initiate new communication attempts.
Thus, there is a need to provide a method for a Completion to a Not Registered/Not available user (CCNReg) service in a SIP based network (IMS, or non-IMS network).
Another aim of the invention is to provide the interworking of CCNReg service to PSTN/PLMN. Interworking will enables this service to be offered to:                a PSTN/PLMN user, when this user makes a communication attempt,                    to a SIP/IMS user who is not registered currently, or if the “presence” status of the called user indicates unavailability of the called user;            or to a PLMN user when the originating PSTN/PLMN and the terminating PLMN networks are connected only via an IMS/SIP/VOIP network in between.                        an IMS/VOIP/SIP user, when such a user makes a communication attempt to:                    a PLMN user who is not present/not reachable/not available,            or to an IMS/VOIP/SIP user when the originating and the terminating IMS/SIP/VOIP networks are connected only via an PSTN/PLMN in between.Note that in case of calls between IMS/SIP users, the calling and called users can belong to the same IMS/SIP network or different IMS/SIP networks.                        
FIGS. 1 to 4 illustrate various examples of interworking:
FIG. 1 shows an example in which a PSTN/PLMN User A calls a IMS/SIP User B.
FIG. 2 shows an example in which a PSTN/PLMN User A calls a PLMN User B, originating and terminating networks being connected via an IMS/SIP network.
FIG. 3 shows an example in which an IMS/SIP User A calls a PLMN User B.
FIG. 4 shows an example in which an IMS/SIP User A calls an IMS/SIP User B, originating and terminating networks being connected via a PSTN/PLMN network.